Ultrasonic flow sensors have been employed for a number of years for performing intraoperative or extracorporeal blood flow measurements. Intraoperative flow measurements are typically conducted to monitor blood flow in various vessels during vascular, cardiac, transplant, plastic and reconstructive surgery. Extracorporeal blood flow measurements are made externally of the patient during procedures in which the patient's blood is removed for treatment, such as for example, ECMO, hemodialysis, CP bypass and CAVH procedures, by measuring the blood flow as it passes through sterile tubing. These types of flow sensors measure volume flow in the vessels or tubing by employing ultrasonic transit-time principles of operation.
Typically, an ultrasonic flow sensor includes a flow probe that houses two ultrasonic transducers and a fixed acoustic reflector. The transducers are positioned on one side of the vessel or tube under study and the reflector is positioned midway between the two transducers on the opposite side of the vessel or tube. In operation, an electrical excitation is applied to one of the transducers which emits a plane wave of ultrasound. This ultrasonic wave provides full flow illumination of the vessel or tube under study, and first intersects it in the upstream direction. The wave then bounces off of the acoustic reflector, intersects the vessel again, and is finally received by the other transducer which converts it into an electrical signal. From the signal, the flow sensor derives an accurate measure of the "transit time" for the ultrasonic wave to travel from one transducer to the other. This same transmit-receive cycle is then repeated, but with the transmitting and receiving functions of the transducers reversed so that the liquid flow under study is bisected by an ultrasonic wave in the downstream direction. Again, the flow sensor derives an accurate measure of transit time from this transmit-receive sequence.
During the upstream measurement cycle, the ultrasonic wave travels against flow, and this increases the total transit time by a flow dependent amount. Similarly, during the downstream cycle, the ultrasonic wave travels with the flow, thus decreasing the total transit time by the same flow-dependent amount. By subtracting the downstream transit time from the upstream transit time, the ultrasonic flow sensor can therefore determine the fluid volume flow through the vessel or tube.
The foregoing technique works well on straight vessels or tubing carrying a flow without non-axial flow disturbances such as vortexes. Unfortunately, if the vessel or tubing is curved about its longitudinal axis or the flow has vortexes, flow disturbances are created so that the flow volume is no longer uniform across the vessel's cross section.
With a conventional flow probe, if a curvature is applied to a straight vessel or tube, the angles which the two wave paths make with the longitudinal axis of the vessel or tube both either increase or decrease. This results in an inaccurate flow measurement since the change in the time of flight of the ultrasonic wave imparted by the fluid flow is directly proportional to the velocity of the flow and the cotangent of the angle between the flow direction and the ultrasonic wave path. Since the curvature of the vessel or tube either increases or decreases both of these angles, the flow measurements are either too high or too low. As a result, conventional ultrasonic flow sensors are not suitable for use on curved blood vessels or near vessel side branches as are commonly found in humans and other animals, especially small animals.
The size of the flow sensor's probe also inherently places limitations on its use. For example, the size of the probe may prevent use of the flow sensor when measurements need to be made on very short blood vessels, such as those which are close to body organs. This is especially of concern in applications involving very small animals, such as mice or rats. The probe size in conventional ultrasonic flow sensors is inherently limited by the fact that the probe must incorporate at least two spaced ultrasonic transducers which must be aimed across the flow path at an angle of less than 90.degree. relative to the flow direction in order to add a flow-dependent component to the transmitted waves. Another factor that also becomes especially important when flow measurements are made on small animals, such as mice or rats, is the flow sensitivity of the probe. With very small diameter blood vessels (e.g., 1 mm or less), the flow rates are also particularly small, requiring that probe flow sensitivity not be sacrificed if accurate measurements are to be made,
Referring to FIGS. 6-7, a prior art probe 90 is known which has a reflector 80 with a V-shaped side profile. Reflector 80 includes first and second reflecting surfaces 82 and 84 positioned at a 90.degree. angle with respect to each other. This arrangement of reflecting surfaces 82 and 84 results in an incident ultrasonic wave from a first transducer 94 being split into separate waves as they reflect partly from first reflecting surface 82 and partly from second reflecting surface 84 at the same time. The plane of the ultrasonic wave as it leaves first transducer 94, as shown by the solid arrowheads, thus changes during reflection, as shown by the hollow arrowheads, before being received by a second transducer 96. One disadvantage is that tube 98 must be positioned within the "V" so that the flow information is adequately determined. This limitation in positioning explains why the relative size of the body and reflector of probe 90 is large in comparison to the maximum tube size being measured. An additional disadvantage is that this probe does not provide a sufficient insensitivity to spatial flow non-uniformities, thus limiting the accuracy of the probe.